1. Field of the Invention
This invention pertains to the field of antennas. More particularly, this invention pertains to dipole antennas and to a novel single-element dipole antenna that radiates and receives multiple frequencies.
2. Description of the Prior Art
If we could visualize the radiation surrounding us as we conduct our daily lives, we would be truly amazed. There is radiation of virtually every frequency imaginable swimming about us in a huge cloud as we work and play. Much of this radiation has to do with radio and television broadcasting. This invention concerns the reception of television radiation or broadcasting and how it can be captured using newer, smaller and more efficient equipment.
Television broadcasting has been around since the 1940's. It was given a significant boost after the United States began orbiting satellites about the earth that receive television broadcasts from point sources on the earth's surface and then re-broadcast the same signal over a wider area for capture by dish antennas and cabled to various households. Huge television dish antennas sprung up along side many houses and apartment complexes for private use and television cable companies were created for disseminating cabled satellite broadcasts to various households. The result was that television has become a major aspect of everyday life both in the United States and abroad.
Original satellite television broadcasting began on what is known as the "C" band, i.e., a frequency range in the 4 gigahertz (GHz) band. The satellites that broadcast television signals to the ground are parked in geosynchronous orbits and spaced 2.degree. apart from each other. Each of these satellites can have two dozen or more transponders each of which transmits a television signal at a standard frequency or channel. In order to increase isolation between transponders transmitting at a given frequency on the adjacent satellite, the transponders are arranged so that they are orthogonally polarized or cross-polarized, that is, their "E-fields" are at right angles to each other. This requires that the antenna feed must be capable of changing its polarization each time the receiver's channel is changed or when the receiving antenna is moved to an adjacent satellite.
A common feed antenna in television viewing incorporates the dipole radiating element. A dipole antenna is a simple resonant antenna consisting of two substantially equal arm sections of conductive material extending co-axially in a straight line in opposite directions from each other. It is called a "resonant" antenna because it creates a natural undamped frequency that is equal to, or very close to, the frequency or a divisional portion of the signal inducing electrical currents in it.
In reality, the standard dipole feed antenna for satellite dish antennas is about 11/2 inches long and is housed in a short section of a wave guide terminated at one end by a conductive floor forming a cavity therein called a "cavity", that looks much like a coffee cup. Formally it is defined as a cylinder, open at one end, comprising conducting walls and serving as a resonator for electromagnetic waves. However, no wave guide or cavity is required for a more general dipole application. The cavity, with the dipole antenna at its center, is set at the focal point of an earth station with the dipole antenna facing toward the reflector. The dipole antenna receives the satellite radiation reflected from various parts of the dish and passes it down a "balun" to a processing unit for introduction into a cable leading to a television set. A "balun" is a term given to a short piece of transmission line that matches the impedance (resistance) of the antenna and which transforms from an unbalanced transmission line, such as coaxial cable, to a balanced transmission line such as twin lead transmission line that feeds the dipole. The various television satellites separate their channels by polarizing the odd and even channels orthogonal (perpendicular) to each other.
To handle the different polarizations of adjacent stations, the dipole antenna is made to rotate, first to one position for alignment with one channel, then to another position for alignment with another channel. The rotation can be as much as 180.degree. but usually is about 90.degree.. It is this rotation that has caused one of the big problems in TV reception.
To effectively rotate, there must be separation between the central mast of the dipole antenna and the support equipment into which the incoming signal is fed; or so the industry thought. Accordingly, there is provided on all dish antennas a rotation device that causes the dipole antenna to physically rotate about a base shaft. The incoming signal must bridge the gap between the antenna mast and the base shaft and this is accomplished by a standard rotation coupling. The incoming signal is quite weak, having travelled over many thousands of miles from the earth station to the satellite and then back to the dish, and is very susceptible to being further weakened by the slightest electrical mismatch caused by imperfect mechanical fit.
Unfortunately, the rotation coupling of the dipole antenna provides this mechanical interference. Despite refinement in design, the coupling and its rotational driver not only degrade the incoming signal but produce a certain amount of base noise that effectively lessens the net signal passed to the processing equipment down the line. The signal is greatly weakened upon reaching the discriminating and processing equipment and results in a weak TV picture.
If that were not the only problem, a new form of TV signal is now being broadcast from recently orbited satellites. This new form is call "Ku" band radiation. It is of higher frequency, in the 12 Ghz band, which means the wave length is shorter. It is also linearly polarized meaning that the antenna must be rotated to capture the signal efficiently.
The main problem with dipole antennas is that they have been thought of as limited to only one frequency band. There has been much effort expended to increase the ability of dipole antennas to receive more frequency bands. U.S. Pat. Nos. 4,125,840; 4,410,893; 4,460,877; and 5,229,782 have been issued for this purpose, however, they all have one major drawback and that is they are multi-element. This means that if one wishes a dipole antenna to pick up other bands of radiation, one must hang other components on the dipole to effect this result. For instance, U.S. Pat. Nos. 4,125,840 and 4,410,893 call for so many additional elements, in order for the dipole antenna to pick up additional frequencies, that it begins to resemble a Christmas tree with many ornaments hung about its exterior. Any time one adds more elements to an antenna, the antenna becomes far less sturdy and is affected by natural occurrences, such as weather, wind, rain, etc., that normally wouldn't affect the basic or single element antenna. In addition, more elements means more chances the antenna will move out of alignment and require additional tuning in order to remain efficient.
In addition, in the field of cellular telephone communications there is a need for such a wide band dipole antenna to allow operation over a wide band one or more of which may be assigned to a particular cellular telephone.